Hi @Amit Dimri ,
You could bulk set the language for all users in O365 by using following PowerShell command.
get-mailbox -ResultSize unlimited | Set-MailboxRegionalConfiguration -Language 1033
Note: The Language ID is a number that corresponds to the correct language type. For example “1033” is English (United States).
For Reference: Office 365: Bulk set Language and Time Zone for all users in Office 365
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